The present invention relates to a method and an apparatus for dewatering and squeezing material in form of sludge, sediment, suspensions such as paper pulp, peat, etc.
Roller presses, disc presses, wire belt presses and screw presses are used today for dewatering and pressing materials. The three first-mentioned types are in relation to their capacity large and expensive machines, have high operation costs and are unsuitable for purifying plants and small industries. In this connection it has therefore been necessary to use some form of screw presses, however existing constructions have several disadvantages and limitations.
Enclosed drawing illustrates in FIGS. 1 and 2 two different basic principles for such screw presses.
FIG. 1 illustrates a conventional press having a sieve mantle in combination with diverging screw body and an adjustable throttle device at the outlet of the press.
When dewatering a material of the kind mentioned above the press usually is fed with a pumpable inlet concentration, meaning a range of 2% to maximum 6% dry substance. It is desirable that the concentration after the press shall be the highest possible, preferably within the range of 35% to 45%. Such a dewatering requires a compression of the material in the order of magnitude 1:10.
However, the press according to FIG. 1 has a maxiumum compression ratio, counted as transport volume per thread inlet/transport volume per thread outlet, in the order of magnitude 1:2 and by experience maximum 1:2,3. This means that the press must be dimensioned for the incoming volume and for that reason the end portion, where the squeezing work takes place, must be made with large dimensions. For obtaining a dewatering, for example, from 4% up to 40% concentration with a screw press having a compression ratio, for example, 1:2,3 a strong throttling must take place at the outlet of the press. This results in a compression backwards in the press resulting in friction and unnecessary energy consumption. Moreover, such a throttling in the end portion causes that the material receives tends to rotate with the screw, whereby the entire press can be blocked due to overloading.
Another disadvantage with this known screw press is, that upon incoming low material concentration, particularly when a large play exists between thread top and sieve mantle due to worn screw threads, a cloth formation consisting of fibres on the inlet side of the sieve mantle is obtained in the inlet portion. In this type of press there are no possibilities for readjustment of the play between thread top and sieve mantle.
The fact that the final squeezing in this known screw press takes place at an unecessary large diameter, which is determined by the inlet volume for the material, it is also accompanied by the disadvantage of a large moment on the screw for feeding/compression work resulting in a high energy consumption.
Due to the fact that the press must be dimensioned after the incoming volume, the disadvantage accordingly exists that the press must have large diameter and also large length. Since in a screw press considerable radial loads exist, the construction of a press shown in FIG. 1 is expensive and the operation costs are high.
The known screw press illustrated in FIG. 2 has in the outlet portion a somewhat converging sieve mantle in combination with a converging screw and at the outlet an axially operating throttle device in the form of a reciprocating piston. In this construction it is possible to achieve sufficient compression ratio. The main problem of this screw press is that the distance between screen body and sieve mantle is large at the outlet and squeezed water cannot penetrate through the thick fibre cake but is encased at the screw body. This results in that the discharged material becomes varying in concentration. Moreover, problems arise due to the fact that the water collected at the center shaft is compressed and "pushes" the material out with sharp water jets through the outlet. Due to the geometry of the device it is here difficult to arrange dwatering both radially outwards through the sieve mantle and radially inwards through a perforated center shaft.